The interior of a popcorn kernel consists primarily of a hard, corneous starch. When this hard material is heated, the starch becomes plastic; however, due to the almost spherical shape of the kernel and its strong hull, the grain of popcorn retains its shape when heated until internal pressure caused by vaporization of moisture in the grain exceeds the strength of the hull. At this stage, the hull explodes or "pops" and the plasticized starch expands. This expansion has a refrigerating effect which cools the plastic material almost instantaneously and "freezes" it in the well-known blossom shape of popped corn.
For effective expansion of popcorn or other cereal grains, the rate of heating of the grain is critical. For popcorn, the heating rate should be sufficient to bring the grain to popping temperature in a period of 1.5 to 2.5 minutes. If the grain is heated too rapidly, the temperature at the center of the grain remains too low for popping and only the outer portion of the grain is expanded, producing a small popped grain with a gritty and incompletely popped center. Conversely, if the grain is heated too slowly, the moisture within the grain may be driven out before the grain is hot enough to expand, resulting in small, tough, popped grain.
Two heating techniques are commonly used for popping the corn. In one method, the grain is exposed to a bath of hot popping oil which surrounds the grain and provides a rapid heat transfer to the grain. This "wet" popping method, although thermally efficient and relatively simple, coats the popped grain with the oil, changing its flavor. An extremely stable and indigestible oil must be used to withstand the high temperatures (400.degree.-500.degree.F.) necessary to this process. Only about half of the expensive oil is delivered with the corn; the rest is lost by vaporization or sticks to the pan and to unpopped waste corn. Where the popped corn is to be coated with a flavoring, such as butter or caramel, the oil may interfere with the coating process.
In the second method, known as "dry" popping, each grain is heated in the optimum period of time while the grain is tumbled in a rotary cylinder to insure even heating. When the kernel pops, it is promptly conveyed away from the source of heat. All imperfectly popped corn is subsequently delivered to a waste bin. This dry popping method eliminates both the cost and the taste of the oil used in "wet" popping and produces consistently larger kernels.
Previously known dry poppers have usually utilized a gas flame as the heat source. The necessity for rapid heating of the popcorn or other grain has dictated the positioning of the heat source directly below the expansion chamber or cylinder containing the corn, for reasons of efficiency and economic practicality, the corn being heated primarily by convection. For the same reasons, the dry popping process has required the direct application of heat to the corn or other grain; to this end an expansion chamber of perforated metal or of small mesh wire screen, in which the corn can be agitated during the popping process, has been utilized.
A primary problem encountered in conventional dry popping apparatus is the fire hazard that is created while the corn is being simultaneously agitated and heated, and the related possibility that popped kernels of corn may ignite during the short time interval in which they are exposed to the flame beneath the popping chamber. Bits of chaff accompanying the corn and tiny tips of the corn which break off during agitation fall through the perforations in the expansion chamber, burning as they pass through the flame or onto the heater. These burning particles also produce an additional minor possibility of fire in the waste receptacle, which is located below the heater and which also contains partially popped kernels of corn dumped into the receptacle following previous popping operations.
Conventional electrical resistance heaters could be substituted for the gas heaters customarily used in dry popping equipment. If this is done, however, the plastic nature of the falling waste, when exposed to high heat, causes a build-up of material on the resistance heating elements. As a consequence, the electrical heaters rapidly lose efficiency or burn out prematurely. If heat lamps are employed, instead of resistance heaters, the burning waste coats the lamps, with resultant loss in efficiency, or may even cause them to break. Thus, electrical heating elements, when substituted for conventional gas heaters, are neither efficient or economically practical. Furthermore, conventional heat lamps of the required high heat capacity are excessively expensive and the replacement costs are prohibitive if used in this manner. At present, gas-fired dry popping apparatus cannot be used in many locations because of fire protection codes. For example, fire codes have precluded the use of dry poppers in theaters for many years. The same prohibition apply in many large shopping centers. Thus, wet popping has continued in use in many applications despite the improved product and increased efficiency that can be achieved with the dry popping process.